The present invention relates to an apparatus for measuring an electromagnetic field distribution using an electron beam as a probe, and in particular, it relates to an apparatus for measuring an electromagnetic field distribution in an extremely small area using a focused electron beam.
A differential phase contrast method for measuring an electromagnetic field distribution in an extremely small area inside a magnetic thin film is known to the public, in which a specimen is irradiated with a focused electron beam and the direction and the magnitude of a deflection (Lorentz deflection) of the beam caused by the magnetic field in the specimen when the beam is transmitted through the specimen is detected by an electron beam position detector. In this method, since an electron beam can be focused to a very fine beam (diameter: in the order of several nanometers), a magnetic field distribution can be measured with a very high resolution by scanning a specimen with the focused electron beam with a high precision. The differential phase contrast method is explained in, for example, "Materials Science and Engineering, B3, 355-358 (1989)".
In an apparatus for measuring a magnetic field distribution using the above-mentioned differential phase contrast method, a deflection phenomenon of an electron beam caused by a magnetic field is utilized, but the deflection of an electron beam is caused by not only a magnetic field but also by an electric field. Therefore, in a case where the influence of an electric field is large, or where a detailed magnetic field distribution is to be investigated without ambiguity, it is necessary to separate the effect of the electric field from that of the magnetic field.
In general, when the direction of incidence of an electron beam onto a specimen is reversed, in the Lorentz force given to an electron beam by an electromagnetic field, the direction of a force due to the electric field is not changed, but the direction of a force due to the magnetic field is reversed. Therefore, it is possible to determine the electric field distribution and the magnetic field distribution separately at a point at which the electron beam is transmitted, by the combined analysis of the direction and the magnitude of deflection of the electron beam caused by the Lorentz force when the electron beam is transmitted through a specimen in one direction, and on the direction and the magnitude of deflection of the electron beam when it is transmitted through the specimen in the opposite direction to the above.
As described, for example, in the Physical Review, B34, 3397-3402 (1986), the separate measurements of an electric field and a magnetic field based on the principle described above is possible. In the above case, after a specimen is observed under a holographic interference microscope which can detect the deflection of an electron beam caused by a Lorentz force, the specimen is taken out of the microscope and turned over, and then it is put back under the microscope again for observation. The separation of the contribution of an electric field from that of a magnetic field is performed by comparing the observation results obtained before and after the turnover of the specimen. In the method as described above, however, a rather long time is needed for the observations, and the observation area may shift between the 2 cases, that is before and after the turnover of the specimen.
Another example of a method for reversing the direction of incidence of an electron beam onto a specimen is to prepare two sets of measuring systems composed of two electron sources and two electron beam position detectors, which are disposed to have opposite directions of incidence of electron beams onto a specimen. In this method, however, the apparatus is necessarily bulky and expensive, and further its operation is complicated; thus, the apparatus cannot be considered practical.